3.3 - Transport in plants

Question 1

Why do plants need transport systems?

Answer 1

To move water and sugars around their bodies

Question 2

How is water absorbed and transported in the plant?

Answer 2

Water is absorbed in the roots of the plants and transported up the plant to the leaves through the
xylem

Question 3

How is sugar made and transported in the plant?

Answer 3

Sugars are made in photosynthesis, manufactured in the leaves and are transported up and down to provide fuel for respiration

Question 4

How is oxygen transported in plants?

Answer 4

Through simple diffusion

Question 5

What is the process of the movement of water into plants and up towards the xylem?

Answer 5

-Mineral ions are absorbed by active transport in the
roots
-This causes water to move in the roots via osmosis
-The water travels across the cells after the root hair
cells down a WP gradient in one of three ways: via the
apoplast pathway, the symplast pathway and vacuolar pathway
-When the water reaches the casparian strip, this then
causes the apoplast pathway to be blocked so the
water must go through the vacuolar or symplast
pathway
-After the water has passed the casparian strip, mineral ions are actively transported into the xylem
-This causes a high concentration of ions in the vascular bundle which lowers the water potential, water molecules then passively move into the vascular bundle containing the xylem
-This causes a high root pressure in the xylem which causes the water to be drawn up the stem towards the leaves

Question 6

How are root hair cells adapted to increase water uptake?

Answer 6

Root hair cells have:

• A high surface area
• Large amounts of mitochondria to provide the ATP for active transport as it is quite a metabolically active cell

Question 7

What is the apoplast pathway?

Answer 7

• Water moving through the pores of the cell walls between the cells
• This pathway is the fastest way for water to move through the roots

Question 8

What is the symplast pathway?

Answer 8

-Water moving inside, through the cytoplasm of the
cells
-This crosses the plasmodesmata from cell to cell

Question 9

What is the vacuolar pathway?

Answer 9

• Water moving through the cell but through the tonoplast into the vacuoles specifically
• This still crosses the plasmodesmata between cells

Question 10

What is the casparian strip?

Answer 10

An impermeable ring around the cell that is made of suberin

Question 11

What is the cross section of a root? (The star)

Answer 11

• Circular outside membrane
• Outside of the endodermis is the cortex
• Inner ring called the endodermis, contains the casparian strip and many mitochondria for active transport of mineral ions into the vascular bundle
• Inside the endodermis is the pericycle with meristem tissues
• Medulla inside the endothelium surrounding the phloem and the xylem
• Phloem around the star shaped xylem
• Xylem star shaped in the centre of the root

Question 12

What is the purpose of the casparian strip?

Answer 12

-The casparian strip blocks the apoplast pathway so the water is forced into the symplast or the vacuolar
pathway
-The casparian strip also blocks the mineral ions that are actively transported into the vascular bundle from leaking back into the cells
-The casparian strip maintains the high concentration of mineral ions in the vascular bundle, allowing water to passively move into the xylem via osmosis

Question 13

What are the features of the xylem?

Answer 13

-Made up of dead cells due to lignification which kills the cells
-Lignin in cell walls making them strong, stop it from collapsing and makes the cell walls impermeable
-Can have bordered pits that allow lateral movement
out of xylem cells to supply the tissues with water
-Continuous “tube”, as they are fused end to end
-Flexible because of spiral or annular lignin structure
-No cell contents to allow space for water movement

Question 14

How are xylem cells adapted to compliment the properties of water to allow the transport of water up the plant?

Answer 14

• The xylem is a continuous tube with no cell contents or divisions
• This allows the cohesive and adhesive properties of water to travel up the xylem cells

Question 15

What is cohesion?

Answer 15

• Cohesion is how the molecules are stuck together
• This means that as water exits through transpiration it causes the molecules below to be drawn up the xylem as they are held together by cohesion

Question 16

What is adhesion?

Answer 16

• Adhesion is how the water molecules stick to the sides of the xylem walls
• This allows the molecules to travel up the walls via capillary action

Question 17

What is capillary action?

Answer 17

The cohesive and adhesive properties of water working together to transport the water molecules

Question 18

What is the process of the movement of water in plants up the xylem and out the leaves?

Answer 18

• The water travels up the xylem in the stem via capillary action
• Water passively moves out the xylem via bordered pits by osmosis
• Water then evaporates from the surface of the cell to the air spaces
• The built up water vapour in the air spaces then diffuses put the stomatal pores into the surrounding environment

Question 19

What is the cross section of a stem? (Not the star)

Answer 19

• Outer collenchyma ring
• Cortex surrounding the vascular bundles
• Vascular bundles in a ring around the stem
• The sclerenchyma closest to the edge
• Phloem on the outside of the vascular bundles (Closer to the surface)
• Cambium in the middle (Meristem)
• Xylem on the inside of the vascular bundles
• Medulla in the centre of the stem cross section

Question 20

What is the structure of a leaf?

Answer 20

-Waxy cuticle on surface = stops water loss out of the
cell
-Upper epidermis = transparent to let the light through to the photosynthetic cells
-Palisade mesophylls = contains lots of chloroplast in the cell and a vacuole that pushes the chloroplast to the outside of it so that it maximizes light absorption
-Vascular bundle = xylem on the top and the phloem on the bottom
-Spongy mesophyll with air spaces that are full of dense air (Packed of moisture)
-Stomata with stomatal pores = allows water to evaporate out of the stomatal pore
-Guard cells = regulates transpiration

Question 21

How does water move within the leaf structure?

Answer 21

• Water will leave the xylem in the vascular bundle down a water potential gradient
• It can then passively move to palisade mesophyll cells were it is needed for photosynthesis
• Or it can move through the spongy mesophyll and evaporate to the air spaces
• The dense water vapour will then diffuse out the stomatal pore into the environment
• The moist air may then be blown away by the wind, washing over the leaf
• The diffusion of water outside of the stomata, powers the process of transpiration

Question 22

How do guard cells regulate transpiration?

Answer 22

Guard cells regulate transpiration by:

• Closing the pores if there is a lack of water in the plant (In times of water stress), as the water must be retained
• Or opening if the plant has enough water to photosynthesise, this causes the water to leave and CO2 to enter via the stomatal pore

Question 23

What is the purpose of transpiration?

Answer 23

• Supplies water for photosynthesis in the leaf cells
• Supplies water to keep the cells turgid
• Cools down the leaves when they are in the sun for too long

Question 24

What apparatus is used to measure transpiration rate?

Answer 24

A potometer

Question 25

How is a potometer used in an experiment?

Answer 25

• Cut a shoot from a plant underwater
• Seal the cutting into the potometer underwater
• Allow an air bubble to enter the bottom of the potometer as the transpiration begins then put the bottom of the potometer into the water
• Measure the distance the air bubble moves up the potometer towards the shoot over time
• Plot a graph of the distance travelled by the air bubble over time
• This is used to work out the transpiration rate

Question 26

How will validity be ensured in this experiment?

Answer 26

• The plants must be the same
• The cutting must be the same just repeated several times
• The stem must be cut with a sharp knife, underwater to stop air bubbles from entering as they would block the flow of water up the xylem (The cohesion)
• Seal the shoot into the potometer under water

Question 27

How can this experiment be changed to measure the affect of different variables on transpiration rate?

Answer 27

• Light: Place a lamp by the potometer with a heat shield in the middle to prevent heat reaching the plant and to absorb infrared radiation
• Air movement: Place a fan by the potometer, this blows the moist air that sits around the exit of the stomata from diffusion out the leaf
• Humidity: Place a plastic bag round the shoot and the potometer entrance

Question 28

How will the variables being changed affect the transpiration rate?

Answer 28

• Light: Increases rate of transpiration as the guard cells will detect the light and the stomata cells will be open allowing the water vapour to diffuse out
• Air movement: Increases the rate of transpiration as the humid air does not surround the leaf, the concentration gradient is maintained so the air can diffuse out the leaf at a more constant rate
• Humidity: Decreases the rate of transpiration as the surrounding air to the leaf becomes more humid, so the concentration gradient is not as steep. This means that the water vapour will diffuse out the leaf at a lower rate

Question 29

How is the volume of water lost calculate?

Answer 29

Distance x πr2 = the volume of a cylinder so therefore is the same as the volume of water lost

Question 30

What are xerophytes?

Answer 30

• Plants that are adapted for living in hot and dry (Arid) environments
• For example: Cacti or marram grass

Question 31

What are some adaptions of xerophytes?

Answer 31

• Curled leaves = Protects the inner surface containing stomata from wind to retain the humid air around the stomata (To prevent transpiration)
• Stomata in sunken pits (In the lower epidermis) = Further distance for water vapour to exit the stomata
• Extra thick waxy cuticle to prevent water loss
• Hairs inside the pits minimise air flow that could carry water away

Question 32

What are hydrophytes?

Answer 32

Plants that are adapted for living in wet environments

Question 33

What are some adaptions of hydrophytes?

Answer 33

• The stomata is on the upper surface as the lower surface is covered in water. This allows the plant to do gas exchange with the air on the upper surface
• Air spaces in the leaves to provide buoyancy and cause them to float to the surface of the water
• The air spaces and the long stem also allows oxygen to diffuse down from the surface and the air spaces to the roots or the bottom of the plant

Question 34

How is the sugar in plants made?

Answer 34

From photosynthesis in the palisade mesophyll of the

leaf

Question 35

How is the phloem adapted to help move sugar and assimilates up and down the plant?

Answer 35

• Living cells, no nucleus or organelles just cytoplasm
• Must be alive as metabolic processes must happen in the companion cell connected to the phloem
• Contents of the cell is removed, this gives room for mass flow
• Many plasmodesmata (sieve plate) between the cells to help mass flow
• Companion cell is connected at the plasmodesmata, this keeps the phloem alive as it does all the metabolic processes

Question 36

What do the companion cells do?

Answer 36

• Companion cells help load and unload the sugar
• Into the phloem cells when made in the leaves &
• Out the phloem when it is going to be stored in a sugar storage organ for example as starch in a potato
• The companion cells also do all the metabolic processes such as: Protein synthesis and respiration, to keep the phloem alive as an active cell

Question 37

What is the process of loading sugar into the phloem from the companion cells?

Answer 37

1) H+ ions are pumped out, this creates a gradient as there are more H+ ions out of the companion cell
2) H+ ions flow back into the companion cells using a cotransport protein and carrying sugar
[Cotransport of H+ ions and sucrose into the
companion cell]
3) There is a high concentration of sugar in the companion cell so the sugar passively diffuses through the plasmodesmata into the phloem
4) Water follows the sugar into the phloem via osmosis
5) This creates a high hydrostatic pressure at the phloem in the source

Question 38

What is the source of a plant?

Answer 38

• Where sucrose is actively LOADED into the phloem

- Water enters the phloem following the sucrose

Question 39

What is the sink of a plant?

Answer 39

-Where active UNLOADING of the sucrose out of the phloem occurs, this means the sucrose can be converted into starch to maintain the concentration
gradient
-Water leaves the phloem following the sucrose

Question 40

What is mass flow?

Answer 40

• The movement of water and sucrose in the phloem from the source to the sink
• This movement is caused by the high hydrostatic pressure that is maintained due to the movement of water into the phloem at the source and out of the phloem at the sink

Question 41

What is mass flow caused by?

Answer 41

-The loading of the sucrose in the source and the unloading of sucrose at the sink
(Water going in the phloem at the source and out of the phloem at the sink)

Question 42

What is transpiration?

Answer 42

The loss of water vapour from the leaves

Question 43

How do cacti photosynthesise?

Answer 43

• Cacti are adapted to not have any leaves to reduce water loss
• This means that they photosynthesise using their stem
• This is why the stem is protected with spikes to prevent herbivores from eating the plant to get the water inside

Question 44

Why does the sink of a plant have a high hydrostatic pressure?

Answer 44

• The sucrose leaves the phloem at the sink and it is converted straight into starch for storage
• The starch being stored at the sink is insoluble so it doesn’t affect the water potential in the phloem
• This maintains a high hydrostatic pressure as the water potential stays high due to the lack of sucrose in the phloem
• Due to the high water potential in the phloem the water moves to the xylem in the roots via osmosis to build a high root pressure
• This maintains the high hydrostatic pressure in the sink of the phloem and the high pressure in the roots